Q-switched laser device for altering surfaces

ABSTRACT

A high gain optically pumped Q-switched laser device includes a mask having cutout portions defining a given pattern disposed in the resonant optical cavity defined between end mirrors at opposite ends of the laser material. Each of the end mirrors is equivalent to an optically flat reflecting surface such that only cross-sectional portions in the laser material similar to the pattern are stimulated when the Q of the cavity is restored to emit narrow pulses of output laser radiation. These pulses have high power densities and may be directly radiated to a target surface to alter the surface in a manner corresponding to the given pattern. The alteration may take the form of actual vaporization, heating, chemical reaction, or oxidation of portions of the surface. Thus, carefully controlled engraving type processes or other processes involving a physical alteration of a surface such as the drilling of square or unusually shaped holes are determined by the pattern can be carried out.

United States Patent Rothrock 1151 3,657,510 14 1 Apr. 18, 1972 [54]Q-SWITCHED LASER DEVICE FOR ALTERING SURFACES [72] Inventor: Larry R.Rothrock, Poway, Calif. [73] Assignee: Union Carbide Corporation, NewYork,

22 Filed: Nov. 19, 1970 21 Appl.No.: 91,024

LIGHT PUMP souaceu g Primary ExaminerJ. V. Truhe Assistant ExaminerHughD. Jaeger Attorney-Pastoriza & Kelly [57] ABSTRACT A high gain opticallypumped Q-switched laser device includes a mask having cutout portionsdefining a given pattern disposed in the resonant optical cavity definedbetween end mirrors at opposite ends of the laser material. Each of theend mirrors is equivalent to an optically flat reflecting surface suchthat only cross sectional portions in the laser material similar to thepattern are stimulated when the Q of the cavity is restored to emitnarrow pulses of output laser radiation. These pulses have high powerdensities and may be directly radiated to a target surface to alter thesurface in a manner corresponding to the given pattern. The alterationmay take the form of actual vaporization, heating, chemical reaction, oroxidation of portions of the surface. Thus, carefully controlledengraving type processes or other processes involving a physicalalteration of a surface such as the drilling of square or unusuallyshaped holes are determined by the pattern can be carried out.

11 Claims, 4 Drawing Figures PATENTEDAPR 18 I972 LASER HEAD INTENSITY .R0% O NR H T m0 Q-SWITCIIED LASER DEVICE FOR ALTERING SURFACES Thisinvention relates to lasers and more particularly to a novel high gainoptically pumped Q-switched laser capable of altering surfaces of targetmaterial in accordance with given patterns.

BACKGROUND OF THE INVENTION It is known in the an to provide lightimages by disposing a mask in front of output radiation from a laser.The principles involved are substantially the same as in normal lightprojectors used for projecting slides or motion picture film. In thesedevices, the entire output radiation impinges upon the mask and thetransparent or cutout portions of the mask defining the pattern pass apercentage of the output radiation. While optical imaging of the patternon the mask is possible with this system, it is very difficult toprovide sufficient energy density or power in the image defined by themask to effect operations other than that of simply imaging a picture.Such operations might include, for example, drilling through a material,effecting a welding along a given pattern as defined by the mask, orimprinting clear patterns on specific types of material such as certainplastics.

The two major difficulties in attempting to perform such operations inaccord with a pattern in the mask arises from the fact that, firstsufficient energy or power is not available in the beam after it leavesthe mask because of the eclipsing which cuts down a large part of thetotal power available and, second, if the overall power is increasedsufficiently to attempt to solve this problem, the mask itself is oftensubject to destruction.

In simple light imaging systems, it has been proposed to in corporate amask in the optical cavity of a laser so that the mask itself will notbe subject to destruction. Such a system has been described in US. Pat.No. 3,293,565 issued to W. A. Hardy for a gas laser wherein curved endmirrors are required in order to effect the lasing action together witha suitable lens to project the image from the system. With such a gaslaser, there is not nearly enough output energy in the projected beam toeffect physical alteration of a target surface.

In my co-pending application Ser. No. 51,798 filed July 2, 1970 andentitled LASER DEVICE FOR ALTERING SUR- FACES IN ACCORDANCE WITH GIVENPATTERNS, there is disclosed a laser device in which a-mask defining adesired pattern is placed in the laser cavity but wherein the outputpower or energy density from the laser is sufficient to effect physicalalterations of various surfaces so that drilling and welding operationsor engraving type processes on certain materials can readily be carriedout.

While my above described laser device is highly successful in manyoperations, I have discovered that in attempting to alter the surface ofcertain types of materials in accord with a given pattern, the resultingsurface alteration is not as clear or clean cut as would be desired. Onesuch difficult material to deal with is silicone rubber. Another istetrafluoroethylene resin, commercially produced under the trademarkTeflon. This latter plastic is often utilized as wire insulation andnormally is difficult to write on or otherwise code. First, the surfaceitself when intercepting a laser radiated pattern reacts with the lightin such a manner as not to provide a clear cut imprinting; rather, theimprinting is fuzzy. Second, in the case of small diameter wires, it isalmost impossible to provide a readable code or nomenclature on thelimited surface of the Teflon available.

Other materials such as the coatings on certain phannaceutical productsdo not react well with the laser radiated pattern when generated by thespecific embodiment set forth in my above identified application.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The present inventioncontemplates an improved laser device for enabling successful patternformingon certain materials which cannot easily be treated in accordwith the specific embodiment set forth in my above referral to copendingapplication.

More particularly, the improved laser device, as in my copendingapplication utilizes a high gain optically pumped laser material,preferably a solid state cylindrical crystal. First and second endmirrors define the resonant optical cavity for the laser material, thefirst end mirror being an optical flat with percent reflectivity or anend mirror means equivalent to such an optical flat, and the second endmirror being optically flat and partially transmissive to couple thelaser radiation out of the optical cavity.

A mask having cutout portions or equivalent transparent portionsdefining a given pattern is positioned in the optical cavity in such amanner that only cross-sectional portions in the laser material or roditself similar to the pattern are stimulated to emit output laserradiation. The arrangement is as though a plurality of individual laserrods were oriented to define the pattern, each rod operating at fullcapacity. A Q- switching means is included in the optical cavity so thatvery narrow, high energy pulses may be generated. When the generatedpattern is coupled out of the system, the pulse width and energy densityis such as to provide a desired physical alteration on certain targetsurfaces, such as Teflon, in a clean cut and controllable manner.

In a modified arrangement, as in my co-pending case, the first endmirror means equivalent to an optically flat reflecting surfacecomprises a diverging lens cooperating with a concave reflecting mirrorspaced from the diverging lens and positioned to intercept and returnthe Q-switched laser pulses back through the diverging lens. By thisarrangement, the cutout portions or pattern on the mask may occupy anarea substantially greater than the cross-sectional area of the lasermaterial itself.

BRIEF DESCRIPTION OF THE DRAWINGS:

A better understanding of the present improvement will be had byreferring to preferred embodiments thereof as illustrated inthe-accompanying drawings, in which:

FIG. 1 is a schematic type perspective of a laser device according tothe invention;

FIG. 2 is a schematic elevational view partly in cross-sectionillustrating the manner in which a lens system may be employed with thestructure of FIG. 1;

FIG. 3 is a schematic view of a modified laser device according to theinvention enabling the use of larger mask structures as compared to thatof the device in FIG. 1; and

FIG. 4 illustrates the narrow high energy pulses.

DETAILED DESCRIPTION OF THE INVENTION Referring first to FIG. 1 there isshown a laser material preferably in the form of a crystal rod 10 pumpedby any suitable means such as a helical lamp 11 connected to a lightpump source 12. Suitable first and second end mirror means 13 and 14define a resonant optical cavity for the laser material.

In the embodiment of FIG. 1, the first end mirror means constitutes anoptically flat mirror providing substantially 100 percent reflectivityfor laser radiation while the second end mirror means comprises anoptically flat mirror which is partially transmissive to couple thelaser radiation out of the optical cavity.

In accord with the invention, a Q-switch 15 is disposed in the opticalcavity together with a mask 16 having cutout portions defining a givenpattern 17. The Q-switch may constitute a bleachable dye, electro-optictype or mechanical such as a rotating prism, and enables the generationof high energy, narrow output pulses. The mask, in the particularillustration shown has a cutout pattern in the form of the letter E andthis pattern falls within an area no greater than the cross-sectionalarea A of the laser material 10.

With the foregoing arrangement, and assuming that the laser material islight pumped and Q-switched to effect stimulated emission of giantpulses of radiation, such stimulated emission can only occur atcross-sectional portions of the laser material corresponding to thepattern 17. Thus, there is indicated by the shaded end of the crystalrod 10 as at 18 those cross-sectional portions of the laser materialwhich are actually caused to lase. The resulting system would be similarin function to a series of small laser rods in side by side relationshipaligned to define the letter E, each rod operating at full capacity.

The resulting laser output beam will have a cross-section correspondingto the pattern 17 and when it strikes a target surface 19, it willactually alter the surface as indicated at 20 in a manner correspondingto the pattern. This alteration may take the form of vaporization,heating, oxidation, or chemical reaction.

It should be understood that while the entire laser crystal material 10is light pumped, only those cross-sectional portions corresponding tothe pattern as shown at 18 will actually lase and since the mask istransparent to those portions there is no possibility of deteriorationof other portions of the mask when disposed as shown. Further, some ofthe light energy pumped into portions of the crystal outside the lasingcrosssectional portions will be utilized in effecting the invertedpopulation levels in those portions which are caused to lase therebyresulting in a very high energy density in those portions of the outputbeam defining the pattern.

The provision of a Q-switch in combination with the mask in the cavityconstitutes the heart of the present invention as it enables theimprinting or engraving of a pattern on certain materials whichheretofore could not be successfully marked or have their surfacesaltered by a non-Q-switched laser such as a simple cw or pulsed laser.One such material is Teflon and in the example of FIG. 1, this materialis shown as the target 19 serving as insulation for a wire.

FIG. 2 illustrates a system similar to that of FIG. 1 wherein there isprovided a laser head 21, first and second end mirrors 13 and 14 with aQ-switch l and mask 16 having a cutout pattern 17 disposed in thedefined optical cavity. The laser head 20 may contain the same elementsand 11 as described in FIG. 1, Q-switch, mask, and pattern beingdesignated by the same numerals used in FIG. 1.

In the embodiment of FIG. 2, there is shown an exterior lens system 22positioned to intercept the output laser beam of cross-sectioncorresponding to the pattern 17. This lens may be adjusted to enlarge ormagnify the final pattern when it strikes a target surface or,alternatively, and as shown in the drawing, may focus down the patternto a very small geometry for impingement on the target. The numerals 23and 24 indicate corresponding portions of the pattern on the mask whenthe lens system has reduced the size of the cross-sectional area. Whenthe cross-sectional area is reduced down by the lens system, a smallertargetsuch as a Teflon coated wire 25 may be easily marked by the giantpulses of radiation generated in the Q-switched laser system.

In some instances, it map be impractical to provide a mask with apattern wherein the pattern is confined within an area on the mask nogreater than the cross-sectional area of the laser material. In thisevent, a modified system such as illustrated in FIG. 3 may be used.

With specific reference to FIG. 3, there is shown a laser head 27, firstend mirror means 28 comprising a diverging lens 29 and a concavereflector 30, a second end mirror 31, and a Q-switch 32. The componentsin the laser head 27 and second end mirror 31 may be the same as thecorresponding elements described in FIG. 1. The first end mirror means28, however, while functioning equivalently to an optically flat endmirror such as the mirror 13 of FIG. 1 enables the use of a larger maskpattern than is possible in FIGS. 1 or 2. Such a larger mask is shown at33 with cutout portions 34 and 35 constituting part of a pattern. Thearea of the laser material in the laser head 27 is determined by thediameter of the rod indicated at D2. This diameter may correspond to thediameter D] of the laser rod of FIG. 1. It is important to note that thecross-sectional area of the laser light leaving one end of the laser rodin the head 27 at the point it passes the diameter arrows D2 isprecisely the same as the cross-sectional area of the returningradiation after total reflection from the first end mirror means 28. Thefirst end mirror means 28 comprised of the diverging lens 29 and concavereflector 30 is thus equivalent to an optically flat end mirror asdescribed heretofore.

In FIG. 3, the tagget indicated at 37 is shown as a pharmaceuticalproduct; for example, a coated pill. In this instance, a careful controlof the laser light is necessary to assure a proper reproduction of thepattern on the pill coating. Such proper control is possible with aQ-switched laser since the output pulse frequency and width can becarefully controlled by controlling the Q-switching. The same is true inthe systems of FIGS. 1 and 2. However, the arrangement of FIG. 3 permitsa larger mask pattern to be utilized.

FIG. 4 illustrates the difference in pulse shape when using a Q-switchedlaser as compared to a pulsed laser. Thus, the solid curve 38 representsa relatively wide pulse of radiation of width W2 as would result from apulsed laser, such as in my heretofore mentioned co-pending application.While this type of pulse works well in altering surfaces of manymaterials, it does not provide a clean cut or clear pattern on certainother materials such as Teflon as heretofore described.

The dotted pulses 36 represent substantially narrower pulses of width W1generated by the Q-switching technique. The pulses may be several asshown, or only one. It will be evident that a greater power density isrealizable. Further, it will be clear that by controlling theQ-switching, the number of such narrow pulses per unit time striking thetarget material can be controlled. Highly successful marking ofmaterials such as Teflon as described in FIGS. 1 and 2 or pharmaceuticalcoatings as described in FIG. 3 are thus possible with the Q- switchingtechnique.

OPERATION The operation of the laser device of this invention will beapparent from the foregoing description. Should it be desired to code aTeflon material in accordance with a given pattern, such pattern isformed on the mask such as the mask 16 of FIG. 1 so that onlycorresponding cross-sectional portions of the laser rod 10 will lase.Impingement of the output pulses properly controlled by Q-switching onthe Teflon will thus result in carbonization of the surface to provide aclear imprint of a desired pattern or code.

If it is desired that the altered surface of the target follow a similarpattern but which similar pattern is smaller, the lens system describedin FIG. 2 may be utilized.

Finally, if it is not feasible to provide a pattern which can beencompassed within an area on the mask corresponding to thecross-sectional area of the laser material, the modified structure ofFIG. 3 may be utilized.

In all cases, the very high energy narrow output pulses effect thedesired alteration of the target surface as opposed to mere opticalimaging.

While the mask structures have been described as having cutout portionsdefining the pattern, it will be evident that equivalent patterns can beformed by providing portions that are simply transmissive to theparticular wave length involved. The term cutout portions is meant toinclude any equivalent means of defining patterns on the mask forrealizing the desired ends.

What is claimed is:

1. A Q-switched laser device for altering the surface of a targetmaterial to define a given pattern comprising, in combination:

a. a high gain optically pumped laser material;

b. first and second end mirror means defining a resonant optical cavityfor said laser material, said first end mirror means providingsubstantially percent reflectivity for laser radiation without changingthe cross-sectional area of said radiation at the point it leaves oneend of said laser material and returns to said one end, said second endmirror means being optically flat and partially transmissive-to couplethe laser radiation out of said optical cavity;

. a Q-switching means for switching the Q of said resonant opticalcavity to enable the generation of narrow high energy pulses ofradiation; and

. a mask having cutout portionsdefining said given pattern,

I said mask being positioned in said optical cavity such that onlycross-sectional portions in said laser material similar to said patternare stimulated to emit output laser radiation, whereby an exteriortarget surface may be radiated by said output laser radiation to therebyeffect physical alteration of said surface similar to said givenpattern.

2. A device according to claim 1, including an exterior lens systembetween said second end mirror means and said target surface forchanging the size of the cross-section of said output laser radiationdefining said given pattern whereby a smaller or larger similar patternof altered surface portions can be formed on said target surface.

3. A device according to claim 1, in which said first end, mirror meanscomprises an optically flat mirror, the overall outer dimensions of thepattern defined by the cutout portions of said mask being such that saidpattern falls within an area no greater than the cross-sectional area ofsaid laser material, the

cross-sectional portions in said laser material'that are stimulatedbeing congruent with the cutout portions on said mask defining saidpattern.

4. A device according to claim 1, in which said first end mirror meansincludes a diverging lens for expanding said laser radiation after itleaves said 'one end of said laser material and passes said point; and aconcave reflecting mirror spaced from said diverging lens and positionedto intercept and return the laser radiation back through said expandinglens so that when the radiation passes said point to return to said oneend, it is of the same cross-sectional area as when it left, said maskbeing positioned between'said diverging lens and said concave mirror,whereby the cutout portions on said mask defining said pattern mayoccupyan area substantially greater than the cross-sectional area of saidlaser material.

5. A device according to claim 1, in which said laser materi alcomprises a solid state crystal in the shape of a solid cylindrical rod.

6. A device according to claim 1, in which said exterior target surfaceis Teflon.

7. A device according to claim 1, in which said exterior target surfaceis a pharmaceutical coating.

8. A device according to claim 1, in which said exterior target surfaceis silicone rubber.

9. A process for altering the surface of a material to define a givenpattern comprising: 1 I

positioning, in the resonant optical cavity of a laser containing a highgain optically pumped laser material and having a substantially percentreflective first end mirror means and a partially transmissive secondend mirror means, a mask having cutout portions defining said givenpattern so that only cross-sectional portions in said laser materialsimilar to said pattern are stimulated to emit out put laser radiation;

providing said laser with a Q-switching means for switching the Q ofsaid resonant optical cavity to enable the generation of narrow highenergy pulses of output laser radiation;

producing in said optical cavity laser radiation having the samecross-sectional area upon leaving, being reflected by said first endmirror means, and returning to the end of said laser material; couplingsaid pulses of laser output radiation out of said cavity through saidsecond end mirror means; and

impinging said pulsed output laser radiation on the surface of saidmaterial to thereby effect physical alteration of said surface similarto said given pattern.

10. Process in accordance with claim 9 which additionally comprises;

positioning a lens system between said laser and said surface forchanging the size of the cross-section of said output laser radiation;and

passing said output laser radiation through said lens system prior toimpingement on said surface to effect a smaller or larger similarpattern of altered surface portions on said surface.

11. Process in accordance with claim 9, in which said laser materialcomprises a solid state crystal in the shape of a solid cylindrical rod.

1. A Q-switched laser device for altering the surface of a targetmaterial to define a given pattern comprising, in combination: a. a highgain optically pumped laser material; b. first and second end mirrormeans defining a resonant optical cavity for said laser material, saidfirst end mirror means providing substantially 100 percent reflectivityfor laser radiation without changing the cross-sectional area of saidradiation at the point it leaves one end of said laser material andreturns to said one end, said second end mirror means being opticallyflat and partially transmissive to couple the laser radiation out ofsaid optical cavity; c. a Q-switching means for switching the Q of saidresonant optical cavity to enable the generation of narrow high energypulses of radiation; and d. a mask having cutout portions defining saidgiven pattern, said mask being positioned in said optical cavity suchthat only cross-sectional portions in said laser material similar tosaid pattern are stimulated to emit output laser radiation, whereby anexterior target surface may be radiated by said output laser radiationto thereby effect physical alteration of said surface similar to saidgiven pattern.
 2. A device according to claim 1, including an exteriorlens system between said second end mirror means and said target surfacefor changing the size of the cross-section of said output laserradiation defining said given pattern whereby a smaller or largersimilar pattern of altered surface portions can be formed on said targetsurface.
 3. A device according to claim 1, in which said first endmirror means comprises an optically flat mirror, the overall outerdimensions of the pattern defined by the cutout portions of said maskbeing such that said pattern falls within an area no greater than thecross-sectional area of said laser material, the cross-sectionalportions in said laser material that are stimulated being congruent withthe cutout portions on said mask defining said pattern.
 4. A deviceaccording to claim 1, in which said first end mirror means includes adiverging lens for expanding said laser radiation after it leaves saidone end of said laser material and passes said point; and a concavereflecting mirror spaced from said diverging lens and positioned tointercept and return the laser radiation back through said expandinglens so that when the radiation passes said point to return to said oneend, it is of the same cross-sectional area as when it left, said maskbeing positioned between said diverging lens and said concave mirror,whereby the cutout portions on said mask defining said pattern mayoccupy an area substantially greater than the cross-sectional area ofsaid laser material.
 5. A device according to claim 1, in which saidlaser material comprises a solid state crystal in the shape of a solidcylindrical rod.
 6. A device according to claim 1, in which saidexterior target surface is Teflon.
 7. A device according to claim 1, inwhich said exterior target surface is a pharmaceutical coating.
 8. Adevice according to claim 1, in which said exterior target surface issilicone rubber.
 9. A process for altering the surface of a material todefine a given pattern comprising: positioning, in the resonant opticalcavity of a laser containing a high gain optically pumped laser materialand having a substantially 100 percent reflective first end mirror meansand a partially transmissive second end mirror means, a mask havingcutout portions defining said given pattern so that only cross-sectionalportions in said laser material similar to said pattern are stimulatedto emit output laser radiation; providing said laser with a Q-switchingmeans for switching the Q of said resonant optical cavity to enable thegeneration of narrow high energy pulses of output laser radiation;producing in said optical cavity laser radiation having the samecross-sectional area upon leaving, being reflected by said first endmirror means, and returning to the end of said laser material; couplingsaid pulses of laser output radiation out of said cavity through saidsecond end mirror means; and impinging said pulsed output laserradiation on the surface of said material to thereby effect physicalalteration of said surface similar to said given pattern.
 10. Process inaccordance with claim 9 which additionally comprises: positioning a lenssystem between said laser and said surface for changing the size of thecross-section of said output laser radiation; and passing said outputlaser radiation through said lens system prior to impingement on saidsurface to effect a smaller or larger similar pattern of altered surfaceportions on said surface.
 11. Process in accordance with claim 9, inwhich said laser material comprises a solid state crystal in the shapeof a solid cylindrical rod.